This section provides background information related to the present disclosure which is not necessarily prior art.
A friction clutch assembly or “clutch” of a car or other automobile having a manual transmission is generally located between the engine and the drive train. The assembly normally includes three adjacent annular plates, including a flywheel that is rotatably driven by the crank shaft, a clutch plate (otherwise known as a driven plate), and a pressure plate that is biased by energy storing devices, such as one or more springs, towards the clutch plate and flywheel to clamp the clutch plate between the flywheel and the pressure plate.
The frictional engagement of the coupling faces of the clutch plate with the adjacent rotating coupling faces of the flywheel and the pressure plate allow the clutch plate to transfer power generated by the engine to the remainder of the drive train. To facilitate this frictional engagement, both the coupling faces of the clutch plate are lined with a frictional material that exhibits substantially stable coefficients of static and dynamic friction over a wide range of operating temperatures, including cooler starting temperatures and significantly hotter running temperatures. The frictional material needs to resist wear, be strong enough to withstand frequent heavy impact loading, particularly during starting, stopping and changing gears, and be non-aggressive against the adjacent coupling faces of the flywheel and the pressure plate.
The clutch functions to regulate the power being transmitted from the engine to the transmission and the drive shaft. When the clutch is disengaged when starting, stopping and changing of gears by depressing an associated clutch pedal that moves the pressure plate away from the clutch plate and the flywheel against the bias of the spring(s), smooth slippage is allowed between the engine and the transmission. Conversely, when the clutch is actively engaged by releasing the clutch pedal so that the spring(s) again bias the pressure plate towards the clutch plate and flywheel, slippage is prevented to maximise the amount of torque that is able to be transmitted from the engine to the drive train.
Two factors limiting the performance of conventional clutches are the maximum power or torque that can be transmitted from the engine to the drive train, and the heat generated by the frequent frictional engagement and disengagement between the coupling faces of the clutch plate and the adjacent coupling faces of the flywheel and the pressure plate. These are particularly evident in high performance cars that generate considerable torque, during intentional aggressive slipping of the clutch, and during frequent riding of the clutch by lesser skilled drivers.
The maximum power or torque that can be transmitted from the engine to the drive train is constrained by the total area and the coefficients of friction of the coupling faces of the clutch plate and the coupling faces of the flywheel and the pressure plate, and the clamping force of the spring. The first of these may be addressed by increasing the areas of the coupling faces by increasing the size of the clutch. However, increasing the diameter of the clutch plate requires additional material, not only for the clutch plate, but also the associated flywheel and pressure plate, and the surrounding clutch housing (or “bell housing”). Further, smaller or compact design cars, particularly front wheel drives, may be unable to accommodate a larger clutch and/or associated larger bell housing.
Another option for increasing the power or torque is to stiffen the spring(s) to increase the clamping force acting on the clutch plate. Correspondingly however, this leads to an increase in the force required to depress the clutch pedal that a user may find undesirable. In extreme circumstances, this can lead to problems such as fire wall flex where the clutch pedal is mounted, or even broken linkages to the clutch where the linkages are unable to withstand the frequent increased forces being transmitted.
The second limitation of conventional clutches relates to the heat generated between the coupling faces of the clutch and the adjacent coupling faces of the flywheel and pressure plate that can result in clutch fade. Clutch fade is effectively a loss of friction force as a result of the heating of the frictional material lining the clutch plate, and is generally caused by the heat resulting from the frequent frictional engagement between the clutch plate and the adjacent rotated flywheel and pressure plate increasing the temperature of the frictional material such that it is within the temperature range at which binder and other constituent frictional materials of the clutch plate tend to melt down and vaporize. The vapour becomes trapped between the adjacent coupling faces of the clutch plate and the flywheel and the pressure plate causing the coupling faces of the clutch plate to glide on blankets of vapour resulting in increased slippage of the clutch.
The applicant's International Patent Application No. PCT/AU2005/000921 discloses a previous design of pressure plate having a series of sweeping grooves to improve torque capacity of a clutch assembly. However, the applicant has determined that it would be highly desirable to provide a relatively small clutch assembly having an even further increased power or torque capacity, as well as reduced susceptibility to fade conditions.